It is well known to use photo sensors such as position-sensitive detectors (“PSD”), or photo arrays, to detect a light source for applications such as image cameras, spot light detection, pointing devices for computers, or position measurement. What has challenged the designers, however, is the optical lens which is needed to produce a focused light spot on the photo sensors. An optical lens, as can be appreciated by those skilled in the art, is a delicate, expensive component. The conventional approach is that to achieve useful detection, a focused image of light sources in the environment needs to be projected by an optical lens onto the light sensitive surface of the sensor. For example, in a digital camera, a focused image of an object has to be projected through the optical lens onto the imaging sensors in the camera, where the image sensors may be a charge-coupled device or CMOS imager. If the image is not focused, the camera would produce a blurry image of the object. For such applications, a good quality optical lens is an indispensable element.
For some applications, however, optical lenses present more challenges to the designers than solutions. A slight defect or error in quality, manufacture or implementation, on the part of the optical lens, would compromise its performance, thus rendering the image, or information, useless. To replace the optical lens would incur additional cost, both in labor and parts, which hurts the bottom line.
For other applications, the delicate and expensive optical lens would seem to present an over-engineered solution, since the projection of a somewhat out-of-focus image can still be sufficient, and sometimes essential. In such cases, sharp focusing of the image is actually not necessary. While the sensor of such applications is still position-sensitive, it is not an array of small pixels, as in the case of a conventional imager or camera. Rather, the sensor may possess continuous position sensitivity across its monolithic surface, or the sensor may be comprised of large magnitude sensitive segments. Here, the goal is not to obtain a precise, focused image of the object on the sensor. Rather, it is to obtain a sufficiently focused image, or even a sufficient, yet unfocused, image, to enable the detection of the centroid of the source. In some cases, the correct performance of the sensor depends on having the image of the light source spread across more than one segment of the sensor, thus rendering precise focusing disadvantageous. Under these circumstances, the more expensive optical lenses, which are required for digital cameras or imaging applications, have proved to be less desirable.
Additionally, in some weak signal environments, it is desirable to have a large entrance pupil diameter (“EPD”) on the lens, so as to increase the signal-to-noise ratio. An exemplary weak signal environment is one, as described in the co-pending patent applications, which uses reflected light sources to estimate the position and orientation of a mobile robot. Such an environment also calls for a wider field of view (“FOV”), so that the light sources can be observed from a variety of angles. As one skilled artisan can appreciate, the EPD and FOV turn out to be two conflicting goals: the gain in one dictates a compromise in the other.
Therefore, it would be desirable to have an optical system that can provide an optimized EPD and FOV combination.
It is also desirable to have an optical system that can be implemented with low-cost manufacturing processes and inexpensive components, without the need for precision manufacturing.